The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO 2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to <9.5 m.y. after the Gaskiers glaciation. Thus, despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes, it appears that Earth narrowly escaped a third Neoproterozoic snowball glaciation just prior to the late Ediacaran expansion of metazoan ecosystems.

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The Burin Peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the Precambrian–Cambrian transition. Fossils are abundant and include trace fossils, small shelly fossils, vendotaenid algae, soft-bodied megafossils, and microfossils. The Burin Peninsula is readily accessible and has long been considered a potential area for a Precambrian–Cambrian boundary stratotype.A continuous section through the upper part of member 1 and all of member 2 of the Chapel Island Formation is exposed at Fortune Head, and this section is herein proposed as a global stratotype for the Precambrian–Cambrian boundary. The boundary horizon is located 2.4 m above the base of member 2 of the Chapel Island Formation. This horizon marks the base of the basal Cambrian Phycodes pedum (ichnofossil) Zone and immediately overlies the top of the Late Precambrian Harlaniella podolica (ichnofossil) Zone. Shelly fossils (sabelliditids) first appear a few metres below the proposed boundary. Soft-bodied megafossils, carbonaceous impressions of vendotaenid algae, and organic-walled microfossils occur both below and above this boundary and enhance global correlation with this section.Fossils of the Rusophycus avalonensis (ichnofossil) Zone first appear midway through member 2 (approximately 135 m above the proposed boundary) and occur commonly throughout the upper part of the Chapel Island Formation and the overlying Random Formation. Calcareous small shelly fossils (?Circotheca sp.) appear near the top of member 2 (approximately 400 m above the proposed boundary), and a more diverse Aldanella attleborensis small shelly fossil assemblage characterizes the uppermost strata of member 3 and all of member 4 of the Chapel Island Formation (approximately 550–650 m above the proposed boundary). The lowest trilobites, representatives of the Callavia broeggeri Zone, first appear more than 1000 m above the proposed Precambrian–Cambrian boundary.

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